These are the latest findings from sky surveys by NASA's Chandra X-ray Observatory, whose images of the more energetic part of the electromagnetic spectrum rival those in visible light from the Hubble Space Telescope in their ability to reveal details about the universe's distant past.

"We wanted a census of all the black holes and we wanted to know what they are like," astronomer Niel Brandt of Penn State University told attendees at the American Association for the Advancement of Science annual meeting Friday. "We also wanted to measure how black holes have grown over the history of the universe."

Brandt's team aimed Chandra at a patch of sky in the Northern hemisphere, near the Big Dipper, producing an image called the Chandra Deep Field-North. They also enlarged a previously surveyed patch in the Southern hemisphere, rendering the Extended Chandra Deep Field-South.

Together, the Chandra North and Chandra South surveys required 2.4-million seconds of telescope time - more than 667 hours or nearly 28 days - over a period of two years. The combined images cover a portion of the sky about twice the size of the full Moon.

"We designed (the Chandra surveys) carefully to complement each other, to be sure that we would get consistent results from two different fields," Brandt told SpaceDaily.com. "Otherwise, we'd be worried that what we got wouldn't be consistent overall - and they're quite consistent, within about 20 percent."

When they compared the Chandra X-ray images with optical images of the same parts of the sky taken by the Hubble Space Telescope, they found a total of about 1,400 X-ray sources whose locations corresponded with the locations of 1,400 galaxies, meaning each galactic nucleus contained a super-massive black hole that was emitting X-rays. Extrapolated, that figure translates into about 300 million super-massives populating the sky.

Brandt considers that a secondary discovery secondary, however. More important, he said, is that the X-ray luminosity of black holes seems to have diminished with time, and along a remarkably consistent gradient. The brightness of active galactic nuclei dims along a steady scale according to distances and times from the present.

In other words, whatever energies super-massive black holes exhibit tend to fall within a surprisingly narrow range, depending on when they appeared in the history of the universe. Brandt calls the phenomenon an "anti-hierarchy," and it has thrown current thought about the evolution of the universe into renewed puzzlement.

"We knew quasars had their peak at high red shifts," Brandt said, "but prior to Chandra, we thought that the moderate-luminosity and lower-luminosity galactic nuclei would behave in the same way, and their behavior is completely different."

The rule is, the higher the X-ray luminosity of a super-massive black hole, the higher its red shift tends to be, meaning it governs a galactic center farther away from the Milky Way and older in the history of the universe.

Brandt's findings impressed even other black-hole experts presenting their research at the AAAS meeting. "Fabulous, really fantastic stuff," Jeffrey McClintock of the Harvard-Center for Astrophysics, told SpaceDaily.com. "The most luminous quasars, and hence the most massive black holes, were there at the beginning, while the little punier ones were created much more recently - just the anti-hierarchical thing he was talking about. To me, that's really exciting."

Brandt remains more reserved, however. "I have nagging worries that we could still be missing many heavily obscured active galactic nuclei in the distant universe, and we should go and find them and develop a good demography - really know how many of them are out there," he said.

He said he hopes to mount a new Chandra survey lasting 10-million seconds, or more than 100 days of telescope time. "I would like to start as soon as possible," he added.

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CDF North
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